The past few years have seen an explosive growth in the number of mobile devices such as cellphones, PDAs, and laptop computers. These devices can use a variety of wireless access technologies. These range from wide-area technologies such as GPRS, EDGE, CDMA 1 xRTT, EV-DO, and satellite access, to local-area technologies such as 802.11a/b/g and short-range technologies such as Bluetooth, Zigbee, etc. However, any wireless access technology must make a difficult tradeoff between the coverage of an access point and the capacity available to a user in that access point's coverage area. To offer wireless access in a given geographical area, wide-area wireless access technologies require fewer access points but offer inherently lower per-user capacity. Short-range access networks can offer large per-user capacity, but the capital cost to offer coverage in large geographical areas can be prohibitive. Consequently, no single wireless access technology can be expected to provide ubiquitous, high-bandwidth coverage. For example, high-speed 802.11a/b/g access coverage is typically confined to WLANs inside buildings and public hot-spots. In contrast, lower-speed WWAN technologies such as CDMA 1 xRTT and GPRS provide far wider coverage, although even such technologies cannot be expected to be available everywhere and coverage can be decidedly spotty inside enclosed areas. Although city-wide 802.11 mesh network deployment tries to provide ubiquitous wireless broadband access, the success is limited because of external interference in the 2.4 GHz band, and a reduction in capacity when the multi-hop count is large. Note that, besides this coverage-capacity tradeoff, managed wireless technologies impose limits on the number of simultaneous users in a given geographic area. This may prevent a user from using a network even when it is available.
Fortunately, current and future mobile devices are likely to come equipped with multiple wireless interfaces that can be used either singly or in parallel.